The number and variety of portable and mobile devices in use have exploded in the last decade. For example, the use of mobile phones, tablets, media players etc. has become ubiquitous. Such devices are generally powered by internal batteries, and the typical use scenario often requires recharging of batteries or direct wired powering of the device from an external power supply.
Most present day systems require a wiring and/or explicit electrical contacts to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. It also tends to be inconvenient to the user by introducing lengths of wire. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each being dedicated to a specific device. Although, the use of internal batteries may avoid the need for a wired connection to a power supply during use, this only provides a partial solution as the batteries will need recharging (or replacing which is expensive). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers, having a tight coupling between primary transmitter coil and a secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections to be made. Indeed, it may simply allow a device to be placed adjacent to or on top of the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer standard known as the Qi standard has been defined and is currently being developed further. This standard allows power transmitter devices that meet the Qi standard to be used with power receiver devices that also meet the Qi standard without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi standard is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Standards documents can be found.
The Qi wireless power standard describes that a power transmitter must be able to provide a guaranteed power to the power receiver. The specific power level needed depends on the design of the power receiver. In order to specify the guaranteed power, a set of test power receivers and load conditions are defined which describe the guaranteed power level for each of the conditions.
Many wireless power transmission systems, such as e.g. Qi, supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information that may allow the power transmitter to adapt to the specific power receiver.
In many systems, such communication is by load modulation of the power transfer signal. Specifically, the communication is achieved by the power receiver performing load modulation wherein a loading applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current draw) can be detected and decoded (demodulated) by the power transmitter.
Thus, at the physical layer, the communication channel from power receiver to the power transmitter uses the power signal as a data carrier. The power receiver modulates a load which is detected by a change in the amplitude and/or phase of the transmitter coil current or voltage.
More information of the application of load modulation in Qi can e.g. be found in chapter 6 of part 1 of the Qi wireless power specification (version 1.0).
It has been found that performance of a power transfer system is dependent on how well the power transmitter and power receiver match each other. For example, in many scenarios, the power transmitter comprises a resonance circuit for generating the power transfer signal and the power receiver comprises a resonance circuit for receiving the power transfer signal. In such systems, it is often advantageous for the resonance frequencies to be matched, and in many scenarios, such a matching is desired in order for the system to operate in what is known as resonant mode. Matching of the resonance frequencies to operate in the resonant mode may often maximize power transfer efficiency.
However, due to e.g. component variations, design variations, environmental changes etc., it is not feasible to generate power transmitters and power receivers with very accurately defined resonance frequencies. Rather, the variation in resonance frequencies may be as much as around 10% of the nominal value in many systems. Therefore, it cannot be guaranteed that a given power receiver and power transmitter will have matching resonance frequencies. In order to improve the matching in such systems, it has been proposed to include resonance circuits having variable resonance frequencies.
For example, WO 2013024396 A1 discloses a specific approach for making the resonance circuit of a power receiver adaptable such that it can adapt its resonance circuit 201 to match that of the received power transfer signal. However, although the approach may improve operation in many scenarios, it is not optimal in all circumstances. For example, in many scenarios, it is not practical to adapt the power receivers. Indeed in many systems there may be a large number of deployed legacy power receivers and it may be desired that new power transmitters can also optimize performance for such receivers. Furthermore, the adaptation of the resonance frequency to a received power transfer signal, such as in particular the approach of WO 2013024396 A1, is complex and often requires careful and accurate measurements of potentially weak signals. For example, the system of WO 2013024396 A1 is based on measurements of a capacitor current for a resonating capacitor of the power receiver resonance circuit. However, such measurements are difficult, require dedicated measurement circuits, and may disturb the resonance behavior.
An improved power transfer approach would accordingly be advantageous. In particular, an approach that allows improved operation, improved power transfer, increased flexibility, facilitated implementation, facilitated operation, improved communication, reduced communication errors, improved power control, improved power transfer, reduced measurement needs, increased support for a variety of power receivers and/or improved performance would be advantageous.